When you do the heel flap, you should have 15 rows: the set up row, and 7 repeats of rows 1 and 2.
Worked back and forth over next 22 sts.
Set-up row (RS) K22.
Row 1 (WS) Sl 1, p21.
Row 2 Sl 1, k21.
Rep last 2 rows 6 times more."
That'll give you 7 slipped sts along each edge and they should look somewhat like the video. You can pick up in each slip st and pick up an extra st at the point where the heel is closest to the ribbing. That corner can often leave a hole so it's good to pick up an extra st there anyway and knit it through the back loop to tighten it up and prevent the hole. You could then just increase 2 sts along the heel flap on the next row to give you the 10sts that you need.